1. Field of the Invention
This invention relates to a light measuring device for use with a spot colorimeter or a multicolor radiation thermometer for receiving and converting a plurality of components of light having different wavelengths from analog to digital values to obtain data to be input to a microcomputer or the like.
2. Description of the Prior Art
Conventionally, a luminance meter normally has such a structure that a light receiving element is placed at a focal position of a lens for condensing incident light. In the case of a spot colorimeter of the noncontact type being a photoelectric colorimeter of such a luminance meter type, three light receiving elements having spectral sensitivities x.sub..lambda., y.sub..lambda., z.sub..lambda. are used, and accordingly incident light must be divided into three components.
Here, the spectral sensitivities x.sub..lambda., y.sub..lambda., z.sub..lambda. are sensitivities which conform to a vision of human beings as provided by the CIE (Commission International de l'Eclairage) XYZ colorimetric system.
It is to be noted that chromaticities x, y are determined from tristimulus values X, Y, Z detected by light receiving elements having spectral sensitivities x.sub..lambda., y.sub..lambda., z.sub..lambda., respectively, and are given by following equations: ##EQU1## Meanwhile, a luminance is determined from Y.
As described above, in order for incident light to be received by 3 light receiving elements which have different spectral sensitivities, it is necessary to divide the incident light into three components. In order to divide incident light into 3 components of different wavelengths, conventionally there are two methods including a method (1) to use a half mirror to divide incident light into 3 components of different wavelengths, and another method (2) to rotate a rotary disk having a plurality of filters mounted thereon to divide incident light with respect to time so that individual components of different wavelengths may be received by respective light receiving elements.
However, the conventional methods described above have following drawbacks:
In the case of the former method wherein a half mirror is used, light receiving elements readily undergo bad optical influences from a lens barrel to cause an error in a stimulus value of each light receiving element, and there is a severe restriction over an arrangement that mounting locations of individual light receiving elements must be specified strictly with respect to incident light.
On the other hand, in the case of the latter time dividing method, when incident light varies with respect to time, a ratio between tristimulus values varies, which will also make a factor of an error. Further, the latter method specifically requires a motor for rotating a rotary disk with filters and a motor controller and is thus complicated in structure. Besides, it is also a problem that it is difficult to synchronize analog to digital conversion of data produced by light receiving elements with rotation of the specific motor.
Further, a conventional photoelectric colorimeter is constituted such that when levels of outputs of amplifiers of the variable gain type for tristimulus values X, Y, Z upon measurement of such tristimulus values X, Y, Z are not within an appropriate range, the gains of the variable gain type amplifiers for the tristimulus values are collectively controlled to the same level.
However, in the case of a photoelectric colorimeter of the type just mentioned, following problems are involved with respect to calculation of a luminance (or brightness) Y.
In particular, depending upon characteristics of a light source, the sensitivity of a light receiving element for a stimulus value Y may sometimes be insufficient. In such a case, if gains of variable gain type amplifiers for all of tristimulus values X, Y, Z are collectively controlled to the same level, there is no problem with respect to calculations of the chromaticities x, y, but with respect to a calculation of a luminance (or brightness) Y, there is a problem that a number of figures required for indication on a display device may not be assured and accordingly the luminance (or brightness) Y cannot be indicated in a definitely discernible condition.
Further, for a colorimeter wherein light from an object for measurement is introduced to a plurality of light receiving elements using a lens system, it has been proposed to locate the individual light receiving elements at slightly defocused positions of the lens system (Japanese patent laid-open No. 61-80018). In the prior art just mentioned, the arrangement of light receiving elements and circuit systems is restricted by such a lens system.
In a colorimeter, light from an object for measurement must necessarily be introduced uniformly to three light receiving elements which have different spectral sensitivities for tristimulus values x.sub..lambda., y.sub..lambda., z.sub..lambda.. Thus, it may be advisable to locate an incoming end of a multi-divided or branched optical fiber at an image forming plane of a lens system on which light to be measured is focused by the lens system while light receiving elements are located adjacent outgoing ends of the multi-divided optical fiber. In this instance, in order to make light receiving levels of light receiving elements uniform, preferably the multi-divided optical fiber is of a random specification wherein a large number of optical fiber strands are arranged at random. However, according to the method, the uniformity of levels of received light incident to the individual light receiving elements depends upon randomness in arrangement of optical fiber strands in the multidivided optical fiber, and there is a technical limitation to promotion of the randomness. It is also a problem that such promotion of the randomness will result in rise of the cost.